Switching system for dual-speed electric servo mechanism



Feb. 23, 1954 Al F, NAYLOR ET AL 2,670,456

SWITCHING SYSTEM FOR DUAL-SPEED ELECTRIC SERVOMECHANISM Filed June 30, 1950 BY @5mm ATTORNEY Patented F eb. 23, `1954 SWITCHING SYSTEM FOR DUAL-SPEED ELECTRIC SERV() MECHANISM Arthur F. Naylor, Haddonfield, and Arnold M. Spielberg, Camden, N. J., assignors to Radio Corporation of America, a corporation of Dela- Wavre Application June 30, 1950, Serial No. 171,442

11 claims. 1

This invention relates to servo mechanisms and more particularly to the type of servo mechanisms in which control of the object to be positioned is transferred from one error signal to another. Such systems are sometimes called dual-speed servo mechanisms.

In order to obtain high precision electric transmission of data, a combined high and low speed servo mechanism system is often employed. For example, in such systems it may be desired to position a distant object to correspond to the rotational position of a control element. Two synchro transmitters may be controlled by the element, one being rotated at some multiple of the rotational speed of the other. Similar synchro control transformers are employed mechanically coupled to the load. From these transformers, two error signals are derived, one of which may be proportional to the sine of the angle by which the load or object is displaced from the position of correspondence, and the other of which may be proportional to the sine of an integral number (n) times the angle. Thus the second signal has a greater periodicity than the rst, and specifically n times the first. In'

the prior art, the low speed or directly proportional error signal is commonly used for control so long as the error is greater than about i in The high speed output is used for control when the error is less than this because of its greater accuracy. The magnitude of the low speed error signal is often used for switching control between the two error signals. Such a dual-speed system, for example, is described and illustrated in connection with Fig. 3.5 at chapter 3 of volume 26 entitled Theory of Servo Mechanisms of the Radiation Laboratory Series, published by Massachusetts Institute of Technology.

One of the great disadvantages of such an arrangement is the discontinuity introduced by the switching action. Particularly if the load is one of appreciable inertia, or if a high speed, high power motor is employed, when the switch is actuated and control of the error signals is interchanged, the load is violently actuated. Operation at the time of switching is jerky and imposes great mechanical strains on the system. Such operation is, of course, undesired.

It is an object of the present invention to improve the operation of servo mechanism systems.

It is another object of the invention to improve the operation of the so-called high precision positional transmission systems in which high and low speed error signals are employed.

Another object of the invention is to maintain continuous control of the positioning mechanism in such systems without objectionable discontinuities in the control signals.

ihese and other objects, advantages, and novel features of the invention will be more apparent from the following description when taken in connection with the accompanying drawing in which:

rig. 1 is a block diagram schematically illustrating a preferred embodiment of the invention; and

rig. z is a schematic diagram more specifically illustrating one oI the blocks of Fig. 1.

In accordance with the invention, the low speed and high speed error signals are combined continuously, in amplifiers, with the gain of the ampliner system controlled in accordance with the displacement error to give predominance to the high or low speed error signals for small or large errors respectively.

Referring now more particularly to Fig. l, a load I is to be driven by a motor I2 to a position of correspondence with a control element. The control element is exemplified by the hand wheel ifi. The hand wheel I4 is coupled directly through suitable gearing to the rotor of synchro transmitter Ib and also to the rotor of a synchro transmitter I8 through a step up gearing I9. It will be understood that the rotor of synchro transmitter I8 because of the gearing i9 is rotated through an angle of 1L times, say times, that of the rotor of transmitter I6. The synchro transmitters I6, I8 respectively, are electrically connected to distant synchro control transiormers 2d, 22. The rotors of synchro control transformers 2t, 22 are connected respectively mechanically to the load io, or the output o1' the motor i2. A gearing 24 steps up the rotation of the rotor of transformer 22 by the same ratio as the gearing 2) steps up the rotor oi transmitter I8. The rotors of transformers 2t, 22 are respectively electrically connected by connections 26 and 28 to amplifiers and control circuits 3D. Connection 26 carries an error signal (e1-the so-called low speed error signal) which is dependent upon the displacement between the load and its position of correspondence with the hand wheel I4 (the control element). Connection 28 carries a second high speed error signal e: which, like the signal e1, has zero amplitude when the load I0 is in its position of correspondence with hand Wheel I4. However, error signal e2 has other zero amplitude error responses. The amplifiers and interconnecting circuits 3i) which are to be more specincally described in connection with Fig. 2, assume theiunction of suitably amplifying and combining the; errorv signals er and'ez. When e1 is near zero, e2 is to be amplified and applied as the control signal or at least predominates as the control signal. As the error amplitude of the error signal e1 increases, the..gain

of the amplifier of the low speed"'error signal e1 is increased and the gain applied to the high speed error signal e2 is decreased untilv control rests almost wholly with thelow speed'error sig-- nal e1. The signals e1, e2, at the outputs of the amplifiers, however, are continuously combined rather than by any switching yaction in a dis# crete step or steps. the low speed error signal e1 to the high error speed signal c2 or Vicel versa,depending on the Violent actuation ventional control circuit suitable and whichfmay.

includeramplifying circuits. Control circuit .32 is electricallyconnectedto the motor. l2 to cause the motor to drive the load iii toward positional correspondencewith hand wheelv il.

Referring., now more f particularly to Fig. 2, exemplifying the ampliers and. interconnecting.; circuits Stof-.Fig l, the connections 25, 28 carryingrrespectively the low and high speed signalsei, e2, may lead. to the primaries it and 42, respectvely, of transformers le and d5. One terminal of a secondary i8 of transformer lll i s con nected to the grid 5l) of vacuum tube 52. Vacuum tube 52 ispart oil an amplier stage 515i. The other terminal of secondary 48 is connected toa rectifier 5B comprising a diode 58 and a resistor-v capacitor combination Eil. Theprectier 5&5y is connected to one terminal of; another secondary 6.4l of transformer 4 4. The limiter 62 which in this. instance is simply a gas tube` preventing currentof eitherY polarity from exceeding a sub-l stantialy Xed amplitude, is connected across the secondary'tli. Theisecondary 613 is grounded at its center tap. The other terminal of secondary 64 is -also connected across the limiter 62, thence i through a capacitor |73 to another rectifier 66.

including a diode biased by a supply at 'l l, and a resistor-capacitor combination 65. Rectifier 66 is connected through a network IG@ to the grid 68 .of vacuum tube it which is part of an ampli# fier stage l2. The grid 63 issupplied with the high speed error signal e2 from. the secondary E4 of transformer dt. VThere are also connected across transformer le, two resistors it, la, the junction between which isconnected to rectiiier 6%. Resistors 'it and i3 form a non-linear com bnation in a manner andl forapurpose that appears hereinafter. The output from ampliiiers 54 and 72 is taken across load resistors and S2 respectively throughblocking capacitors 8d' ancl respectively and through resistors 83 and 90 respectively. The resistors 38 and l are con-V nected at a. junction point 92- from which the control'signal istaken across a resistor-lill.` Re- Control is thus shifted from sistor 94 is small in value with respect to resistors 88 and 63. The three resistors 88, 9@ and 94 form a summing or addition circuit in a manner well known to the art. Each of grids 5@ and 63 is supplied With suitable bias voltage through grid resistors 9S and Sarespectively.

ln operation, let it be assumed initially that the 10W speed error signal e1 is of considerable amplitude and represents a displacement of the load from its Aposition of correspondence of, say 10 or 15; The lowfspeed error signal e1 is then properly supplied to amplifier 54 because the signaleixis comparatively large, and rectied bias from the rectiiier 5t overcomes the bias from resistort. The limiter 82 prevents rectiers 5t and G5 from supplying bias voltages which are too large for the respective ampliiiers lli and 72. It will be noted that rectier 5'6 provides a gain control voltage which, being applied to the grid 5@ of tube 52, tends to increase the gain ofamplier till with increasing amplitude ci the'lcw speed error signal c1. this respect; thcgrid-liacts lilreca; gain control element for amplier 5d; Similarly, gridg acts as av gain, control element for ampliiier E Zescept that the. rectifier 65 rectiiies in apelar-ity to decrease the-y gain of ampliner; 'liwith increasing low speed error signal ci. .Eias at 'il prevents the rectifying circuit t6 from decreasing the gain ofv aniplier-l .when the amplitudeof e1 isrbelovw 1 a. specified level. Capacitor lit is a'blockingz capacitorLinserted so that the D, C.,bias at 1 i Vis isolated from ground andtheresistor HA is in-Y sertedso that the A. C. signal isY isolated from. ground. initially, it may be vassumed that the. erroi` signal'gis sufficiently large so that the volt agerectied in rectifier 5S completely cuts oil? the amplifier l2. Accordingly,th e vlow speed error signal ei is ampliiiedA by amplifier 54, as stated aboveh and thel signalV thus` ampliiied is substan.- tially the solewcontrol` signal output which' is fed4 to the control circuit S2 .ofY vFig,A l which controls. the outputor` the motor'. Nowfsupposethat as. the loadl lll of Fig. l approaches Vpositional:cor--A respondencewith `hand wheel it; vthelovv speed: error signal ker is reduced'. Accorolingfly,V tliegainA control voltage )from rectifier 55 applied to grid. 5@ is dereasediihei atreiilieri5-is einde-. creased. Being in opposite polarities these rectified voltages as they decrease simultaneously, cause one amplier 5d to decrease in gain and the other amplifier 72 to increase in gain.Y The result is that amplification of the low speed error signal ci' is now reduced and that of the high speed error "signal e2 is'increased; 'The outputs of Ythe, two ampliiiersA are added' across the resistor "94;

the circuit `cozrmrising'resistors 8, Si?, and'Sll -conc stitutinga5wel11known`adding'circuit; l

Normally, it' istdesire'dthat atjso'me particular" angular deviation',v say"2.5, oi' the lload 1from its" positional*correspondencev with the controlele-- ment 'that the high' speed error signalez have* complete control."v vV'Ihen by theY time the low" speed error signalv er has the an'iplitude assoelated with such angulardeviations, the bias 'aprA plied to ampliner ,et through' resistor 'Semay be',` assumed sunicientto cutoff 'that amplier, and` thata'pplied'from rectifier insucient to in4` crease the gain of 'ampliiijer' 55 to" an operative.v value. Similarly; .as the positional correspond? enceY isdecreasedjthat is the displacement is inf creased,y amplifier i2 may be out off at a'desiredj' value, by thefbias volta'g'eapplied "from rectiilerfll B.' The cutoivalues may: obviously "be adjusted bysuitably adjusting theb'ias of "ampliers`"`5'41vaffronte andflz, say, at resistors 8Bfand`f98 respectively. :In the transition period, 'when control may be .taken over completely by one .or the other of the `error signals, there 'need be .norsudden switch, or undesired ljerlcing of Vthe equipment. This .is par tieularly true if the .transition is arranged'to 'occur over .suitable spread .of positions of @the load. ...It wiil'rbe understoodv .that vfit is :not .necessary at -any time-to Vfha-veeither signal ccmpletely-outreif, unless so desired.

."Ifh'evcombination 'of signals produced .as the control signal .may tbe writtenA as v1eslci-Htsea where .'Icl, k2, represent variable i'iactors. .For 'the lowf's'peed signal 'el to .have control when the displacement A'is large, the ratio .of 'k2 to ki :must be small andv vice 'versa vfor. 'the high 'speedfsignal e2 to Ahave control. It .is 'Snot essential .that fkr and yk2 both be variable,V asin the :example herein'afforded, ibut either one alone could beheld constant'an'd -the-other varied by omitting'asuitable one of the 'rectifier circuits. It is obvious that t'only their relative values are important `'for the purposes ofthe invention. VThe voltage limiter 6"! is included .so'that the maximum gain control voltage may be suitably llirn'ited. 4This may be omitted.

'it '-is found that 'the vpractice of the -invention as thus far Vdescribed 'improves ythe smoothness of transition between controlof the low and high speed signals. However, to have the proper characteristics 'for -very smooth transfer, 'larger tubes or special *gain controls, 'or both may be required. Afurther "improvement in smoothness of the control transition is effected by making non-linear the network |239. A preferred form is to choose "the resistor 18 Yto be ka non-linear impedance .or resistor of the type commonly known as thyrite as made .by General Electric Company-of a silicon compound. This resistor has a characteristic that when a voltage E is applied across it, the current `I is I =lcE, where n may have-a value of 3 to 2 andJc is a constant. Accordingly, as e2 increases from zeroithe resistance of resistor 18 and the output of the 'network Hill (secondary. 14, vandresistors 15 and 18) to grid 68 rises sharply and then slowly levels oil with increasing values of e2 from zero upward. ''Ihe rectifier bias at "H is preferably so cho'seirth'at the Y.smoothing isxaccomplished by the network |00 when the load is near the position of correspondence but so that the rectified control signal biases oif grid 68 for large deviations from correspondence. Thus with increasing positional error of the load, in effect, the ratio Ici/k2 is rst comparatively small. As the positional error increases k2 increases more slowly. The total error signal characteristic is one which increases very rapidly for values near zero error, with a steep cross-over, and then tends to flatten out. This characteristic assists in giving the desired sharp cross-over and smooth transfer of control.

It will be noted that it is possible for the system to stabilize with equilibrium at a false position with the load 180 away from the desired position of correspondence. This false positioning may be avoided by making the ratio between the high and low speed error signal gearings an odd number, or by using other standard means of avoiding such positions as are known to the art, which do not require switching, e. g., the generation of blocking signals to block the high speed signal loop at the false position. The concern in the present invention is with the great accuracy afforded by the high speed signal without the 'use of discrete switching. :It will 'be derstood that various details ':could be #changed without departing from `the invention. For :example, the triode tubes-shownare illustrative. lThey are shown .in .order lto more directly .and clearly explain the vinventionWithout unnecessary :circuit details. Itfmay tbed'esirable ito ruse ,ilters appropriately v.placed fas .is :customary .finfservo systems, and 'also the usual feedback loops. .However, such additional or optional features Care not described .and illustrated, `because .they are .not n pai-tof the invention, .although they may beiused in the same-system .with 'the invention.

It will Jbe apparent that we have fdevelope'd .a new and novel servo-controlcircuit, in'uwhich low speed and high speed errorsignalsare combined'to provide a composite control signal. 'The composite control signal iscombined continuously, with `one or the other low or high speed signal predominating in rthe composite signal depending on the relative error. .The result :is accomplished by controlling the gain applied to each of two amplifiers for the .high .and low speed error signals respectively iin aaccordance with the error. Further :improvement inayfibe had by the use of non-linear elements Aiii-:coinbination with vsuch 'gain control. vIt vJillIber-apparent that by the present invention, .a system may be built which is of very high Lgain and great accuracy and without v*objectionable features.

` What is claimed is:

l. A dual speed servo mechanismfsystem for bringing an object into positionalcorespondence with a control'element comprising lmea-ns 5to lde rive a first errorv signalfdependentonthe .'d-isplacement of said object from a position -o'f correspondence with said element, means to derive a second high vspeed error A'signal-of greater positional correspondence :periodicity than said first signal, -means to 'combine said signalscontinuously 'into acontrol signal'comprising separate first and second ampliiiersrespectively for said rst and second error signalsfrneans to control the respective gains Aof the 'two V'ampliilers the one with respect -to the vlvother dependent upon the displacement of said object, said gain control means comprising two circuits eachihaving a rectifyingmeans distinct from said amplifiers and each connected to receive said first error signal and each having a gain control voltage due to rectification by said rectifying means responsive to said first error voltage, one said circuit being connected to apply its gain control voltage to said first amplifier to increase the gain thereof with increase of amplitude of said first error signal, the other said circuit being connected to apply its gain control voltage to said second amplifier to decrease the gain thereof with increase of amplitude of said rst error signal, and means to combine continuously the outputs of said amplifiers to provide a control signal, and a motor electrically connected to receive said control signal and mechanically connected to said object and responsive to said control signal to drive said object toward positional correspondence with said control element.

2. The system claimed in claim 1, said means to derive said error signals comprising synchro control transformers.

3. The system claimed in claim 1, the outputs of said ampliers being combined additively.

4. A dual speed servo mechanism system for bringing an object into a position of rotational correspondence with a rotatable element comprising a one-to-one ratio synchro transmitter mechanically coupled to said element, a step-up gear ratio, a second synchro transmitter coupled through said step-up gear ratio to said first element, a synchro control transformer mechanically coupled to said object and electrically coupled to. said iirst transmitter, a second synchro control transformer mechanically coupled through a step-up gear ratio of the same ratio as said first step-up gear ratio to said load and electrically coupled to said second synchro transmitter, iirst and second amplifiers coupled respectively to receive first and second erroi` signals from said first and second transformers, irst and second rectiiiers connectedto receive said first error signal, said first rectifier being connected to a gain control element of said rst amplifier to increase the gain thereof with increase of amplitude of said first error signal, said second rectifier being coupled to a gain element of said second amplier to increase the gain thereof with decreased amplitude of said rst error signal, means to additively combine the outputs of said ampliers, a motor, and means to control the direction of rotation and torque of said motor in response to said additive signal to reduce said additive signal, said load being connected to be driven by said motor.

5. The system claimed inclaim 4, further comprising a peak voltage limiter, said first and second rectiiiers being connected tov receive said iirst error signal through said limiter.

6. The system claimed in claim 4, the coupling of said second synchro control transformer to said second amplifier being through a non-linear network.

7.` The system claimed in claim 4, the coupling of said second synchro control transformer to said second amplifier being through a further transformer having a secondary, and a nonlinear resistor network connected across said further transformer secondary.

, 8. The system claimed in claim 7, at least one of the resistors of said resistor network being of the type known as thyrite.

9. A servo mechanism system for bringing an object into positional correspondence with a control element comprising means to derive a first error signal dependent on the displacement of said object from a position of correspondence with said element, means to derive a second high speed error signal having a positional correspondence periodicity an integral number of times that of said first error signal, means to combine said signals into a composite control signal comprising separate iirst and second amplifiers respectively connected to receive said iirst and second error signals, gain control means comprising first and second rectiiiers connected to receive said iirst error signal, said rst rectifier being connected to said first amplifier to increase the gain thereof with increase of amplitude of said rst error signal, said second rectiiier being connected to said second amplifier to increase the gain thereof with decrease of amplitude of said first error signal, and means toV combine continuously by addition the outputs of said amplifiers, and a motor electrically connected to receive said composite control signal and mechanically connected to said object and responsive to said composite control signal to drive said object toward positional correspondence with said control element.

10. The system claimed in claim 9, further comprising a control circuit including amplifying circuits connected between said addition combining means and said motor.

11. The system claimed in claim 9, one of said amplifiers being connected to receive its respective error signal through a non-linear network.

ARTHUR F. NAYLOR. ARNOLD M. SPIELBERG.

References cites in the ni of this patent UNITED STATES PATENTS Number Name Date 2,409,970 Agins Oct. 22, 1946 2,446,532 Edwards Aug. 10, 1948 2,511,863 McCoy June 20, 1950 2,560,337 Fouassin July 10, 1951 2,561,654 Eller July 24, 1951 2,614,237 Goertz Oct. 14, 1952 2,620,441 McCoy et al Dec. 2, 1952 FOREGN PATENTS Number Country Date 373,253 Italy July 22, 1939 

